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Comprehensive Guide to Halogeno Compounds and Their Reactions

Dive into the intricate world of halogeno compounds, focusing on their structures, including halogenoalkanes and halobenzenes. Understand the mechanisms of nucleophilic substitution reactions (SN1 and SN2) and elimination reactions (E), along with the factors influencing their rates. Explore steric effects, leaving group stability, and the role of solvents. The guide also outlines synthetic applications, such as the formation of nitriles and alkenes, detailing reaction conditions and results. A valuable resource for understanding the chemistry of halogenated compounds.

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Comprehensive Guide to Halogeno Compounds and Their Reactions

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  1. Halogeno-compounds Chapter 33

  2. R R H R C R R C H R C H X X X Structures Halogenoalkanes: X bond to sp3 carbon 1o Primary 2o Secondary 3o Tertiary

  3. X Structures Halobenzene: X bond to benzene, sp2 carbon

  4. Reactions of Halogenoalkanes • Two major types: • Nucleophilic Substitution (SN) • Elimination (E)

  5. H R C+ H X- H R C H + X- Nu Nucleophilic Substitution (SN) :Nu- • Polar C-X bond • C+ is attacked by :Nu- • C-X bond is broken to • give out X-

  6. H H - HO- C+ Br- HO C Br H H CH3 CH3 H HO C + Br- H CH3 Bimolecular Nucleophilic Substitution (SN2) If C is chiral, completed stereochemical inversion.

  7. H - HO C Br H CH3 OH- + CH3CH2Br CH3CH2OH + Br- Bimolecular Nucleophilic Substitution (SN2) Rate law: Rate = k [OH-][CH3CH2Br] (Bimolecular, 2nd order)

  8. R R C+ Br- C+ + Br- R R R R (sp2 , trigonal planar) R R -H+ C+ + H2O: C OH R R R R Unimolecular Nucleophilic Substitution (SN1)  (rds)

  9. R3C---Br R3C---OH2+ R3C+ + Br- + H2O R3C-Br + H2O: R3C-OH + HBr Unimolecular Nucleophilic Substitution (SN1) Rate law: rate = k [R3CBr] (1st order, Unimolecular)

  10. Relative rates of SN1 and SN2

  11. Factors affecting relative rates Structure - Steric Factor The size of atoms or groups at/near the reactive site affects SN2. Bulky groups (-R) at the C-X site slow down SN2 reaction.

  12. Factors affecting relative rates Structure - Stability of carbocation R3C+ > R2CH+ > RCH2+ > CH3+ (R group is e- donating) Stable carbocation favours SN1 mechanism.

  13. Factors affecting relative rates

  14. Factors affecting relative rates Effect of nucleophile SN2 Strength and concentration have effect RO:-> :OH- > ROH > H2O: SN1 No effect

  15. Factors affecting relative rates • Effect of leaving groups • Relative rate of substitution • C-I > C-Br > C-Cl • Explanation : Bond energy • C-I 238 C-Br 276 C-Cl 338 • (*exp.1 p.235)

  16. Factors affecting relative rates • Effect of solvent: • Polar solvent stabilize the carbocation • and hence favour SN1 reaction • Increase in polarity: • CH3COCH3 << R-OH < H2O

  17. Synthetic applications Nitrile Formation ethanol, reflux R-Br + KCN  R-CN + KBr H+ 1.LiAlH4 R-CN  RCOOH  RCH2OH 2.H2O (Increase carbon chain length by one carbon)

  18. Synthetic applications Formation of C-O bond R-Br + NaOH  ROH R-Br + RO-Na+  ROR Formation of amine RI + NH3 R-NH2

  19. H H H H C C C H C H H H H X + H2O + X- Elimination HO:-

  20. H H C C H H H X Competition between SN and E E SN Nu:- Good Nu:- are also good B:- (SN always competes with E)

  21. Conditions favour E • Highly substitutedhaloalkanes is more • likely to undergo elimination (Steric Effect) • Favor SN • 3oRX 2oRX 1oRX • Favor E

  22. Conditions favour E 2. Use less polar solvent e.g. 75% ethanol + 25% water is better than 25% ethanol + 75% water Polar solvent favors the formation of highly concentrated charged particles. T.S. of SN2 reaction is Nu-….R….X- is more concentrated than Nu-…H – C - C….X-

  23. 45oC (47%) (53%) NaOH CH3CHBrCH3 CH3CH=CH2 + (CH3)2CH-OC2H5 (or OH) 100oC C2H5OH, H2O (36%) (64%) Conditions favour E 3. Higher temperature and prolonged refluxing Breaking of C-H bond and C-X bonds require greater Activation Energy.

  24. C2H5OH CH3 (19%) 25oC C Br CH3 (CH3)2C=CH2 CH3 (93%) C2H5O-/C2H5OH Conditions favour E 4. Stronger base: RO- > ROH

  25. Applications of Elimination Preparation of Alkenes e.g. C2H5O-Na+/C2H5OH C2H5Br  C2H5OC2H5 + CH2=CH2 heat 99% 1% C2H5O-Na+/C2H5OH (CH3 )2CHBr  C2H5OCH(CH3)2 + CH2=CHCH3 heat 21% 79%

  26. Applications of Elimination Preparation of Alkenes e.g. C2H5O-Na+/C2H5OH (CH3)3CBr  (CH3)2C=CH2 heat 100%

  27. Applications of Elimination Preparation of Alkynes e.g. Br2 CH3CH=CHCH3  CH3CHBrCHBrCH3 C2H5O-Na+/C2H5OH  CH3CCCH3 heat

  28. Uses of Halogeno-compounds Please refer to Section 33.6 on p.253

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